W 352 Understanding How Turfgrass Herbicides Work Gregory K. Breeden, UT Extension Specialist, Turfgrass Weed Science James T. Brosnan, Associate Professor, Turfgrass Weed Science Javier Vargas, Research Associate, Turfgrass Weed Science Department of Plant Sciences Herbicides are chemicals that inhibit normal other agricultural systems. Developing weed plant growth processes resulting in death of management programs utilizing herbicides that susceptible weeds. The processes by which employ different mechanisms of action is critical to herbicides kill weeds are called modes of action while both preventing and managing herbicide resistant the location where these effects occur at the cellular weeds. It is recommended to rotate herbicides that level is termed the site of action or the mechanism of employ different mechanisms of action as often as action. The Weed Science Society of America (WSSA) possible, as well as implementing cultural practices uses a numerical system to group herbicides by that maximize turf competition and limit weed mechanism of action. A more detailed description of encroachment. Table 2 lists single active ingredient this numerical system is presented in Table 1. herbicides used in turfgrass by mechanisms of action Continual reliance on herbicides that employ the group number. These WSSA group numbers can also same mechanism of action has led to the evolution of be found on most herbicide labels as well (Figure 1). herbicide-resistant weeds in turfgrass, as well as in Figure 1. The Weed Society of America (WSSA) uses a numerical system to group herbicides by mechanism of action. The WSSA group number can be found on most herbicide labels. Table 1. Mechanisms for action of herbicides used to control weeds in turfgrass. WSSA Action Mechanism of Action Description Group 1 Acetyl CoA Carboxylase Inhibit synthesis of fatty acids required for building new cell (ACCase) Inhibitor membranes 2 Acetolactate Synthase Inhibit the acetolactate synthase (ALS) enzyme involved in (ALS) Inhibitor synthesizing branch chain amino acids such as isoleucine, leucine and valine 3 Microtubule Inhibitor Inhibit microtubule structure and function during cell division 4 Synthetic Auxin Act similar to endogenous auxin (IAA) causing uncontrolled cell division and growth that damages vascular tissue 5, 6 & 7 Photosystem II Inhibitor Inhibit proper electron flow during the light reactions of photosynthesis (i.e., photosystem II) 8 & 16 Lipid Biosynthesis Inhibit synthesis of fatty acids and lipids Inhibitor 9 EPSP Synthase Inhibitor Inhibit production of aromatic amino acids (tryptophan, tyrosine and phenylalanine) required for protein synthesis 10 Glutamine Synthetase Inhibits conversion of glutamate and ammonia to glutamine Inhibitor 14 Protoporphyrinogen Inhibit the PPO enzyme involved in chlorophyll and heme Oxidase (PPO) Inhibitor synthesis 15 Long Chain Fatty Acid Inhibit very long chain fatty acid synthesis Inhibitor 17 Specific Site Unknown The mechanism of action for these herbicides remains unknown, but could potentially be nucleic acid inhibitors 21 & 29 Cellulose biosynthesis Inhibit cell wall synthesis inhibitor (CBI) 22 Photosystem I Inhibitor Herbicides accept electrons from photsystem I leading to oxidative stress that damages cellular structures 26 Specific Site Unknown The mechanism of action for these herbicides remains unknown 27 Carotenoid Biosynthesis Inhibit synthesis of carotenoid pigments that protect chlorophyll Inhibitor from oxidative stress Table 2. Single herbicide active ingredients labeled for use in turfgrass. WSSA Action Ex. Trade Mode of Action Mechanism of Action Group Chemical Family Active Ingredient Names Lipid Biosynthesis Acetyl CoA Carboxylase 1 Aryloxyphenoxy-propionate fenoxaprop Acclaim Extra Inhibitor (ACCase) Inhibitor (FOPs) fluazifop Fusilade II Cyclohexanedione (DIMs) sethoxydim Segment Amino Acid Synthesis Acetolactate Synthase 2 Sulfonylurea chlorsulfuron Corsair Inhibitor (ALS) Inhibitor flazasulfuron Katana foramsulfuron Revolver halosulfuron SedgeHammer imazosulfuron Celero iodosulfuron *NA metsulfuron Manor rimsulfuron TranXit sulfosulfuron Certainty trifloxysulfuron Monument Sulfonylaminocarbonyltriazolinone thiencarbazone *NA Imidazolinone imazaquin Image imazapic Plateau Triazolopyrimidine florasulam Defendor penoxsulam Lockup Pyrimidinyl(thio)benzoate bispyribac-sodium Velocity Root Growth Inhibitor Microtubule Inhibitor 3 Dinitroaniline benefin Balan oryzalin Surflan pendimethalin Pendulum prodiamine Barricade trifluralin Treflan Pyridine dithiopyr Dimension Benzamide pronamide Kerb Phthalic Acid DCPA *NA WSSA Action Ex. Trade Mode of Action Mechanism of Action Group Chemical Family Active Ingredient Names Synthetic Auxin Not Well Understood 4 Phenoxy 2,4-D Various MCPP MCPA 2,4-DB Benzoic Acid dicamba Banvel Pyridine Carboxylic Acid clopyralid Lontrel fluroxypyr *NA triclopyr Garlon 4 & 26** Quinoline Carboxylic Acid quinclorac Drive XLR8 Photosynthesis Photosystem II Inhibitor 5 Triazine atrazine AAtrex Inhibitor simazine Princep Triazinone metribuzin Sencor Triazolinone amicarbizone Xonerate Photosynthesis Photosystem II Inhibitor 6 Nitrile bromoxynil Buctril Inhibitor Benzothiadiazinone bentazon Basagran Photosynthesis Photosystem II Inhibitor 7 Urea siduron Tupersan Inhibitor Lipid Biosynthesis Not Well Understood 8 Phosphorodithioate bensulide Bensumec Inhibitor Amino Acid Synthesis EPSP Synthase Inhibitor 9 Glycine glyphosate Roundup Pro Inhibitor Nitrogen Metabolism Glutamine Synthetase 10 Phosphinic Acid glufosinate-ammonium Finale Inhibitor Cell Membrane Protoporphyrinogen Oxidase 14 Aryl Triazinone carfentrazone Quicksilver Disrupter (PPO) Inhibitor sulfentrazone Dismiss N-phenylphthalimide flumioxazin Sureguard Oxadiazole oxadiazon Ronstar Pyrazole pyraflufen-ethyl Octane Shoot Growth Inhibitor Long Chain Fatty Acid 15 Chloroacetamide dimethenamid Tower Inhibitor metolachlor Pennant WSSA Action Ex. Trade Mode of Action Mechanism of Action Group Chemical Family Active Ingredient Names Lipid Biosynthesis Not Well Understood 16 Benzofuran ethofumesate Prograss Inhibitor Not Well Understood Not Well Understood 17 Organic Arsenical monosodium Various methanearsonate (MSMA) Dihydropter-oate Not Well Understood 18 Carbamate asulam Asulox (DHP) Synthase Inhibitor Cellulose Biosynthesis Cell Wall Biosynthesis 21 Benzamide isoxaben Gallery Inhibitor (CBI) Inhibitor Cell Membrane Photosystem I Inhibitor 22 Bipyridilium diquat Reward Disrupter Not Well Understood Not Well Understood 26 & 4** Quinoline Carboxylic Acid quinclorac Drive XLR8 26 Dithiocarbamate dazomet Basamid Carotenoid 4-Hydroxyphenyl-Pyruvate- 27 Triketone mesotrione Tenacity Biosynthesis Inhibitor Dioxygenase (HPPD) Inhibitor Pyrazolone topramezone Pylex Cellulose Biosynthesis Cellulose Biosynthesis 29 Alkylazine indaziflam Specticle Inhibitor (CBI) Inhibitor *NA = Not available alone, but can be found in numerous mixtures. **Quinclorac WSSA action groups are 4 for dicots and 26 for monocots. In turfgrass, multiple active ingredients are often recommended when these materials are applied sold commercially as pre-packaged mixtures to alone (i.e., not in a mixture). Table 3 lists example broaden the spectrum of weeds controlled by a single herbicides labeled for use in turfgrass that contain herbicide application. While these products are very multiple active ingredients and the corresponding useful tools for weed control, they do not replace the WSSA group number for each. Figure 2 is an example need to rotate to herbicides with other mechanisms of how these groups may be represented on the of action because active ingredients in these mixtures product label. are often included at rates lower than what would be Figure 2. Some herbicides labeled for use in turfgrass contain multiple active ingredients from various WSSA groups, as shown on the above label. Table 3. Select herbicides labeled for use in turfgrass that contain multiple active ingredients. Trade Name(s) Active Ingredients WSSA Action Groups 2DQ 2,4-D + dicamba + quinclorac 4 + 4 + (26 or 4) 4-Speed XT 2,4-D + triclopyr + dicamba + pyraflufen-ethyl 4 + 4 + 4 + 14 Anderson’s Goosegrass/Crabgrass oxadiazon + dithiopyr 14 + 3 Blindside sulfentrazone + metsulfuron 14 + 2 Brushmaster 2,4-D + 2,4-DB + dicamba 4 + 4 + 4 Celsius thiencarbazone + iodosulfuron + dicamba 2 + 2 + 4 ChangeUp MCPA + fluroxypyr + dicamba 4 + 4 + 4 Confront, 2D triclopyr + clopyralid 4 + 4 Coolpower, Horsepower, Eliminate MCPA + triclopyr + dicamba 4 + 4 + 4 Dismiss South sulfentrazone + imazethapyr 14 + 2 Echelon sulfentrazone + prodiamine 14 + 3 Escalade 2 2,4-D + fluroxypyr + dicamba 4 + 4 + 4 Freehand dimethenamid + pendimethalin 15 + 3 Last Call fenoxaprop + fluroxypyr + dicamba 1 + 4 + 4 Millennium Ultra 2 2,4-D + clopyralid + dicamba 4 + 4 + 4 Momentum FX2 2,4-D + triclopyr + fluroxypyr 4 + 4 + 4 Negate metsulfuron + rimsulfuron 2 + 2 Powerzone carfentrazone + MCPA + MCPP + dicamba 14 + 4 + 4 + 4 ProDeuce glyphosate + prodiamine 9 + 3 Trade Name(s) Active Ingredients WSSA Action Groups Q-4 Plus quinclorac + sulfentrazone + 2,4-D + dicamba (26 or 4) + 14 + 4 + 4 QuickPro glyphosate + diquat 9 + 22 Solitare sulfentrazone + quinclorac 14 + (26 or 4) Specticle Total indaziflam + glyphosate + diquat 29 + 9 + 22 Speedzone carfentrazone + 2,4-D + MCPP + dicamba 14 + 4 + 4 + 4 Squareone carfentrazone + quinclorac 14 + (26 or 4) Surge, Surezone sulfentrazone + 2,4-D + MCPP + dicamba 14 + 4 + 4 + 4 Tribute Total thiencarbazone + foramsulfuron + halosulfuron 2 + 2 + 2 Trimec